1. Field of the Disclosure
The present invention relates generally to power supplies, and more specifically, the invention relates to power supplies that provide power factor correction.
2. Background
Power supplies are typically used to convert alternating current (“ac”) power provided by an electrical outlet into direct current (“dc”) to supply an electrical device or load. One important consideration for power supply design is the shape and phase of the input current drawn from the ac power source relative to the ac input voltage waveform. The voltage waveform of mains ac sources is nominally a sinusoid. However, due to the non-linear loading that many switching power supplies present to the ac source, the wave shape of the current drawn from the ac source by the power supply is non-sinusoidal and/or out of phase with the ac source voltage waveform. This leads to increased losses in the ac mains distribution system and, in many parts of the world, is now the subject of legislative or voluntary requirements that force power supply manufacturers to ensure the current drawn by the power supply is sinusoidal and in phase with the ac voltage waveform.
The correction of the input current waveform in this way is referred to as power factor correction (PFC) and often requires an input stage to the power supply specifically designed to perform the function of power factor correction. If the input ac current and voltage waveforms are sinusoidal and perfectly in phase, the power factor of the power supply is 1. In other words, a power factor corrected input will present a load to the ac source that is equivalent to coupling a variable resistance across the ac source. The resistance presented as a load to the ac source by the PFC stage is varied as a function of the rms voltage of the ac source in accordance with the power drawn by the PFC stage output load. As harmonic distortion and/or phase displacement of the input current relative to the ac source voltage increase, the power factor decreases below 1. Power factor requirements typically require power factors greater than 0.9 and may have requirements for the harmonic content of the input current waveform.
Common methods to increase the power factor of a power supply include the use of a boost converter or flyback converter to establish an input current waveform close to the ideal sinusoidal shape while being in phase with the ac source voltage.